1. Field of the Invention
The present invention relates to an apparatus for supplying gases or the like for use in the production of semiconductors, chemicals, precision machine parts, etc. More specifically, this invention relates to a parallel divided flow type fluid supply apparatus so configured that when any one of a plurality of flow passages arranged in parallel is opened for fluid to flow, the effect of that operation on the flow rates in other flow passages is minimized.
The present invention also relates to a method of controlling the flow rates of various gases used in an apparatus for supplying gases or the like for use in the production of semiconductors, chemicals, precision machine parts, etc. More specifically, this invention relates to a fluid switchable pressure-type flow control method and a fluid switchable pressure-type flow control system (FCS) in which the flow of various gases can be regulated with high precision by one pressure-type flow control system on the basis of flow factors.
2. Background Art
So-called mass flow controllers are now used in almost all fluid supply apparatuses for manufacturing facilities of semiconductors or chemicals.
FIG. 14 shows an example of the prior art single flow passage-type fluid supply apparatus in which such material gases G are adjusted by a regulator RG from primary pressure to secondary pressure before being sent into the flow passage. The primary pressure is usually a relatively higher pressure and detected by a pressure gauge Po. The secondary pressure is a relatively lower pressure under which the fluid is supplied to the downstream flow passage. The secondary pressure is measured by a pressure gauge P1.
A mass flow controller MFC is installed between valves V1 and V2 for control of the flow. Also provided is a mass flow meter MFM to measure the flow rate. The material gas G is used for a treatment reaction or the like in the reaction chamber C and then discharged by vacuum pump VP through a valve VV.
This single flow passage-type supply apparatus presents no problem with the treatment reaction remaining stable in the reaction chamber C as long as the material gas G is supplied in a normal state with no external disturbances or changes in flow rate.
But a problem is encountered with an arrangement in which material gas G is supplied through one regulator and branched off into two or more flow passages. FIG. 15 shows an arrangement in which the flow of the material gas G from one regulator RG branches off to two flow passages S1 and S2. In practice, a reaction chamber (not shown) is also provided on flow passage S2 and is so arranged that gas reaction may proceed into the two reaction chambers. The same elements or components as in FIG. 14 are indicated by the same reference characters with different suffixes given for different flow passages. Those similar elements or components will not be described again.
An experiment was conducted to study what effect the opening of one closed flow passage would have on the flow of another opened flow passage. In the experiment, the material gas was supplied through flow passage S1 with valve V1 and valve V2 opened and a specific reaction proceeding in the reaction chamber C, while the flow passage S2 remained closed with valve V3 and valve V4 closed. Then, the valve V3 and valve V4 were opened to supply the gas into the flow passage S2 at a specific set flow rate by quickly actuating mass flow controller MFC2.
FIG. 16 shows the time charts of various signals. The instant the valve V3 and valve V4 were opened, MFC2 and MFM2 signals on flow passage S2 overshot to a high peak and then fell to a constant level.
The overshooting or the transient state caused the signals of MFC1 and MFM1 on flow passage S1 to change violently because of a change in pressures P1A, P1B.
This change in turn has an effect on the rate of reaction in the reaction chamber C. The external disturbance from flow passage S2 hinders a steady reaction in the reaction chamber C on flow passage S1. In the process of manufacturing semiconductors, this problem could cause lattice defects in the semiconductor. In etching plasma, the process could be affected. In a chemical reaction, the oversupply or short supply of material gas G could cause finished products to change in concentration. This change could lead to unpredictable problems through xe2x80x9cchaos phenomena.xe2x80x9d However, little transient effect is wrought on upstream pressure Po. This is because of the presence of the regulator RG.
To eliminate the external disturbance indicated in FIG. 16, it is desirable to install regulator RG1 and regulator RG2 on the two flow passages S1 and S2 as shown in FIG. 17. The regulator RG2 could prevent the change in pressure from being felt on the upstream side when the flow passage S2 is suddenly opened. The steady supply of the fluid in flow passage S1 would not be affected. Conversely, the opening and closing of flow passage S1 would have no affect on the side of flow passage S2.
In this connection, the regulator RG is a device to convert the high pressure fluid into low pressure fluid ready for supply to the downstream flow passage. However, the pressure changing device is itself expensive.
The number of regulators RG needed would increase with the number of flow passages. That would make the whole of the fluid supply arrangement complicated and large, sending up the costs.
In the fluid supply apparatuses shown in FIG. 14 and FIG. 15, only one kind of gas is supplied. In practice, however, a plurality of kinds of material gases G are led into the reaction chamber C, one by one or simultaneously, in semiconductor manufacturing facilities.
It is also noted that the mass flow controller is used at almost all semiconductor manufacturing facilities or chemical production plants where the flow rate is required to be controlled with high precision.
FIG. 18 shows an example of the high-purity moisture generating apparatus for use in semiconductor manufacturing facilities.
Three kinds of gases xe2x80x94H2 gas, O2 gas and N2 gasxe2x80x94 are led into a reactor RR through valves V1a-V3a with the flow rate controlled by the mass flow controllers MFC1a-MFC3a. The reactor RR is first purged with N2 gas with valve V3a opened and valves V1a, V2a closed. In the next step, the valve V3a is closed and valves V1a, V2a are opened to feed H2 gas and O2 gas into the reactor RR. Here, H2 gas and O2 gas are reacted with platinum as catalyst to produce H2O gas. The high-purity moisture thus produced is then supplied to downstream facilities (not shown).
The problem is that each mass flow controller has its linearity corrected for a specific kind of gas and a specific low rate range. That is, the mass flow controller cannot be used for other than the kind of gas for which the controller is adjusted.
That is why the mass flow controllers MFC1a to MFC3a are installed for H2 gas, O2 gas and N2 gas, respectively, i.e., one mass flow controller for one kind of gas, as shown in FIG. 18. In a gas supply arrangement as shown in FIG. 18, furthermore, each of the mass flow controllers MFC1a to MFC3a is provided with a standby.
The mass flow controller is expensive and so are replacement parts. That increases the costs of gas supply facilities and the running costs.
Furthermore, if the mass flow controller is not replaced for a new kind of gas and, instead, the linearity is corrected every time a new gas is used, it takes long and it could happen that the operation of the manufacturing plant has to be temporarily suspended. To avoid that, it is necessary to have standby mass flow controllers for different kinds of gases ready in stock.
As set forth above, in case the flow passage from one regulator for regulation of pressure branches off into a plurality of parallel lines and each branch line is provided with a mass flow controller for regulation of the flow rate, then the opening of a branch line can cause a transient change to the other branch flow passages running in a steady state flow. This transient change in turn has an affect on the process in the reaction chamber off the branch line, causing a number of problems.
If each branch line is provided with one regulator to avoid such transient changes, meanwhile, that will make the fluid supply arrangement complicated and bulky, boosting the costs.
Furthermore, a large number of expensive standby mass flow controllers have to be stocked. That increases the costs of gas supply facilities and the running costs.
The present invention addresses these problems with the prior art.
Accordingly, it is an object of the present invention to provide a parallel divided flow type fluid supply apparatus which comprises a regulator RG to regulate the pressure of fluid, a plurality of flow passages S1, S2 into which a flow of fluid from the regulator RG is divided in the form of parallel lines and mass flow controllers DMFC1, DMFC2 for control of the flow rate, one controller installed on each flow passage, wherein the mass flow controller on a flow passage is so set that when the mass flow controller is actuated to open the passage for a steady flow state at a set flow rate, a delay time xcex94t is allowed for the flow rate to rise from the starting point to the set flow rate value Qs.
It is another object of the present invention to provide a parallel divided flow type fluid supply apparatus wherein the delay time xcex94t is adjustable.
It is still another object of the present invention to provide a parallel divided flow type fluid supply apparatus which comprises a regulator RG to regulate the pressure of fluid, a plurality of flow passages S1, S2 into which a flow of fluid from the regulator RG is divided in the form of parallel lines and pressure-type flow control systems FCS1, FCS2, one system installed on each flow passage, the pressure-type flow control system comprising an orifice OR, a control valve CV installed upstream thereof, a pressure detector provided between the orifice and the control valve and a calculation control circuit CCC wherein with the pressure P1 on the upstream side of the orifice set at twice or more higher than the pressure P2 on the downstream side, the flow rate is calculated as Qc=KP1 (K=constant) from the pressure P1 detected by the pressure detector and the difference between the calculated flow rate Qc and the set flow rate Qs is outputted as control signal Qy to the drive DV of the control valve and wherein the flow rate downstream of the orifice is regulated by actuating the control valve.
It is a further object of the present invention to provide a fluid-switchable pressure-type flow control method by flow factor which comprises calculating the flow rate Qc of the gas passing through the orifice according to the formula Qc=KP1 (K=constant) with the pressure P1 on the upstream side of the orifice set at twice or more higher than the pressure P2 on the downstream side, wherein the flow factor FF for each kind of gas is calculated as follows:
FF=(k/xcex3s){2/(xcexa+1)}1/(xcexaxe2x88x921)[xcexa/{(xcexa+1)R}]xc2xd
wherein:
xcex3s=concentration of gas in standard state
xcexa=ratio of specific heat of gas
R=constant of gas
k=proportional constant not depending on the type of gas
and wherein if the calculated flow rate of gas type A is QA, when gas type B is allowed to flow through the same orifice under the same pressure on the upstream side and at the same temperature on the upstream side, the flow rate QB is calculated as follows:
QB=(FFB/FFA)QA 
wherein:
FFA=flow factor of gas type A
FFB=flow factor of gas type B
It is a still further object of the present invention to provide a flow factor-based fluid-switchable pressure-type flow control system which comprises a control valve, an orifice, a pressure detector to detect the upstream pressure therebetween and a flow rate setting circuit, wherein with the pressure P1 on the upstream side held to be about twice or higher than the downstream pressure P2, the flow rate Qc of a specific gas type A can be calculated according to the formula Qc=KP1 (K=constant), wherein the control valve is controlled to open or close on the basis of the difference signal between the calculated flow rate Qc and the set flow rate Qs, characterized in that there is provided storage means for storing the flow factor ratio of gas type A to gas type B (FFB/FFA) which is calculated for each kind of gas as follows:
FF=(k/xcex3s){2/(xcexa+1)}1/(xcexaxe2x88x921)[xcexa/{xcexa+1)R}]xc2xd
Wherein:
xcex3s=concentration of gas in standard state
xcexa=ratio of specific heat of gas
R=constant of gas
k=proportional constant not depending on the type of gas
and that there is provided calculation means in which in case the calculated flow rate of gas type A as reference is QA and when gas type B is allowed to flow through the same orifice under the same pressure on the upstream side and at the same temperature on the upstream side, the flow rate QB is calculated as follows:
QB=(FFB/FFA)QA. 
It is still another object of the present invention to provide a parallel divided flow type fluid supply apparatus wherein the pressure-type flow control system to be installed in any of the flow passages is the flow factor-based fluid-switchable pressure-type flow control system described above.
After extensive study of the working characteristics of the mass flow controller in FIG. 15 and FIG. 16, the inventors found that if the mass flow controller is opened quickly up to the set flow rate level, a large quantity of material gas suddenly flows into flow passage S2. As a result, the pressure P1A in flow passage S1 drops transiently and causes the signal MFC1 and signal MFM1 to undergo a transient change.
To minimize the reflective, transient effect on flow passage S1 of flow passage S2, it is important to let the gas flow into flow passage S2 gradually. That is, after the valves V3, V4 are opened, mass flow controller MFC2 should be so controlled that the flow rate is raised from xe2x80x9c0xe2x80x9d to the set flow rate level in a predetermined time.
That time is called delay time xcex94t. The longer the delay time xcex94t is, the less the transient effect becomes. If this delay time xcex94t can be freely changed, it is possible to cope with transient changes under various conditions.
The delay time xcex94t depends on the size of the set flow rate value Qs, pipe diameter, type of fluids such gas. It is desirable that the delay time xcex94t is determined empirically under various conditions.
The effect on flow passage S1 of flow passage S2 has been described. Conversely, the effect on flow passage S2 of flow passage S1 can be considered the same way. In case the number of flow passages are more than two, the transient effect can be treated the same way.
In case there are a plurality of flow passages and if all the mass flow controllers are to be subjected to time delay control, that can minimize the transient effect of the opening of any flow passage on other flow passages.
Thinking that the mass flow controller had unique characteristics that made it difficult to absorb the transient effect, the inventors also intensively sought some other method not using the mass flow controller.
As a result, the inventors concluded that the mass flow controller cannot absorb the transient effect very well because the controller measures the flow rate on the basis of the amount of heat transfer or heat carried by the fluid, and if the change in flow rate is higher than the flow velocity, the control of the flow rate cannot follow the change in flow rate well.
Thinking that the problem could be solved by using a pressure-type flow control system that could quickly follow the change in flow rate, the inventors decided to adopt the pressure-type flow control system the inventors developed earlier and disclosed under Unexamined Japanese Patent Application No. 8-338546.
This pressure-type flow control system works on the following principle. When the pressure P1 on the upstream side of the orifice is about twice as high as the pressure P2 on the downstream side of the orifice, the velocity of the flow through the orifice reaches the sonic velocity, then the flow rate Qc of the flow passing through the orifice is proportional to the pressure P1 on the upstream side of the orifice. That is given in the equation Qc=KP1(K: constant). In other words, if the pressure P1 on the upstream side alone is known, the flow rate can be immediately worked out. While the mass flow controller determines the flow rate on the basis of heat transfer, the pressure-type flow control system is based on the theoretical properties of fluid. The pressure can thus be measured quickly.
If with a control valve installed on the upstream side of the orifice, the flow rate Qc is worked out by equation Qc=KP1 and then the control valve is controlled to open or close to bring the difference from the set flow rate Qs to zero, the calculated flow rate Qc can be immediately adjusted to the set flow rate Qs. That is made possible by the rapidity with which the pressure P1 on the upstream side of the orifice can be measured. This arrangement can well absorb such changes as shown in FIG. 16.
While working toward development of a fluid supply apparatus using the pressure-type flow control system, furthermore, the inventors hit on a method that allows control of the flow rate without changing the basic setups for a plurality of kinds of gases by using a pressure-type flow control system in place of the traditional mass flow controller.
The pressure-type flow control system (FCS apparatus) the inventors developed earlier is to control the flow rate of the fluid with the pressure P1 on the upstream side of the orifice held at about twice or more higher than the pressure P2 on the downstream side. This FCS apparatus comprises an orifice, a control valve provided on the upstream side of the orifice, a pressure detector provided between the control valve and the orifice and a calculation control unit in which from the pressure P1 detected by the pressure detector, the flow rate Qc is calculated by equation Qc=KP1 (K: constant) and the difference between the set flow rate signal Qs and the flow rate signal Qc is outputted as control signal Qy to the drive of the control valve, characterized in that the pressure P1 on the upstream side of the orifice is regulated by opening or closing the control valve to control the flow rate on the downstream side of the orifice.
The most significant feature of the FCS apparatus is that the flow rate Qc of the gas flowing through the orifice depends only on the pressure P1 on the upstream side of the orifice and can be worked out by the equation Qc=KP1 (K: constant) for one orifice and one gas type.
In other words, if the orifice and gas type are selected and the proportional constant K is set, then the actual flow rate can be calculated with merely the measurement of the P1 on the upstream side of the orifice regardless of changes in the pressure P2 on the downstream side of the orifice. It is the subject of the present invention to determine how the flow rate can be worked out in case the gas type is changed and the pressure found on the upstream side is P1 under the above-mentioned set conditions.
To solve this problem, the meaning of constant K has to be clarified.
First, let it be assumed that a gas flows out through an orifice from the high pressure region to the low pressure region. The law of continuity, law of energy conservation and law of gas state (inviscidity of gas) are applied to the flow pipe. Also, it is presupposed that adiabatic change takes place when a gas flows out.
Further, let it be assumed that the flow velocity of gas flowing out of the orifice reaches the sonic velocity at that gas temperature. The conditions for the sonic velocity to be reached are that P1xe2x89xa7 about 2P2. In other words, the pressure ratio of P2/P1 should not be higher than the critical pressure ratio of about xc2xd.
The flow rate Q at the orifice under those conditions is obtained as follows:
Q=SP1/xcex3s{2/(xcexa+1)}1/(xcexaxe2x88x921){2g/(RT1)xe2x80xa2xcexa/(xcexa+1)}xc2xd
This flow rate Q can be solved as follows:
Q=FFxe2x80xa2SP1(1/T1)xc2xd
xe2x80x83FF=(k/xcex3s){2/(xcexa+1)}1/(xcexaxe2x88x921)[xcexa/{(xcexa+1)R}]xc2xd
k=(2xc3x979.81)xc2xd=4.429 
The physical quantities including the units are as follows:
Q (m3/sec) volumetric flow rate in standard state;
S (m3)=sectional area of the orifice;
P1 (kg/m2 abs)=absolute pressure on the upstream side;
T1 (K)=gas temperature on the upstream side;
FF (m3Kxc2xd/kg sec)=flow factor;
k: proportional constant;
xcex3s (kg/m3)=concentration of gas in standard state;
xcexa(dimensionless)=specific heat ratio of gas;
R(m/K)=gas constant.
Therefore, if it is assumed that the calculated flow rate Qc (=KP1) is equal to the aforesaid flow rate Q, the constant K is given as K=FFxe2x80xa2S/T1xc2xd. It shows that the constant K depends on the gas type, gas temperature on the upstream side and sectional area of the orifice. From this, it is evident that the calculated flow rate Qc depends on only flow factor FF under the same conditions, that is, the same pressure P1 on the upstream side, the same temperature on the upstream side and the same sectional area of the orifice.
Flow factor FF, which depends on concentration xcex3s in standard state, specific heat ratio xcexa and gas constant R, is a factor determined by the gas type only. That is, in case where the calculated flow rate of gas type A is QA, gas type B flows under the same pressure P1 on the upstream side, at the same temperature T1 on the upstream side through the same orifice sectional area, the calculated flow rate QB is given as QB=(FFB/FFA)QA where FFA is the flow factor of gas type A and FFB is flow factor of gas type B.
In other words, if the conditions are identical except for the gas type, the flow rate QB for another gas can be worked out merely by multiplying the flow rate QA by the flow factor ratio of FFB/FFA (FF ratio). Any gas type can be the reference gas type A. In the present invention, N2 is used as a basis as is common practice. That is, the FF ratio is FF/FFN. FFN is the flow factor FF of N2 gas. The physical properties and flow factors of different gases are shown in Table 1.
In calculation of FF ratios, the proportional constant K is eliminated by abbreviation. In calculating FF, therefore, the constant k may be any value. To give k as 1 (k=1) would simplify the calculation. Therefore, the proportional constant k in the respective claims is the higher in arbitrariness.
The authenticity of the aforesaid theory was confirmed in the following procedure. The first step is to flow N2 gas to initialize the FCS apparatus and confirm that the linearity of Qc=KP1 is established under the conditions P1xe2x89xa72P2. The next step is to flow O2 gas and set the P1 on the upstream side of the orifice and at the temperature T1 on the upstream side using the same orifice. O2 gas flow rate Q02 is worked out using the equation Q=FF ratio xc3x97QN, that is, multiplying the N2 gas flow rate QN by the FF ratio of O2=0.9349. Meanwhile, the O2 gas flow rate is compared with the value measured by build up method. It was confirmed that the error was within 1 percent. This shows that the aforesaid theory is correct.
As mentioned above, the flow rate Q of any gas can be calculated from the flow rate QN of N2 gas by the equation Q=FF ratio xc3x97QN.
While the equation QN=KP1 is established, the P1 on the upstream side is proportional to the opening degree of the control valve. With the N2 gas flow rate for an opening degree of 100 percent as QN100, the N2 gas flow rate QN for a certain opening degree is given as QN=QN100xc3x97(opening degree/100). Therefore, the flow rate Q of a gas type can be worked out as Q=FF ratio xc3x97QN100xc3x97(opening degree/100). The FF ratio in this case is FF/FF.
This formula for calculation of the flow rate is useful in finding the actual flow rate Q of gas from the opening degree of the control valve. But it is clear that the formula is identical with the aforesaid equation Q=FF ratio xc3x97QN.